My four year old says this too. I tell her if she is too tired she can go to bed. Usually she changes her min about being too tired. When it comes to getting a toy then thats just tough luck. I think by 5 you could tell your LO that if he says that he will have to sit or lay in his bed for 10 minutes.
Loving wife to Erik - the man who drives me wild and drives me crazy .
Mother to Amelia (12.06), Charlotte (11.08), and Clark (3.11), with #4 arriving in June!